As most pump users know, that when cavitation occurs, it is the result of vapor bubbles collapsing within the impeller passages. It may be within the fluid or on the metal surfaces, and most often does cause vibration in the unit. If severe enough, the vibration will cause the bearings to excessively wear or under extreme circumstances cause catastrophic failure.
Cavitation is also a symptom of a problem such as inadequate NPSHA or some other flow interference which can causes instability in the rotor and causes the rotor to shuttle back and forth resulting in thrust failure. For further information please reference our white paper “Power Plant Cycling: Its effect on NPSHa and the damage it causes to Boiler Water Circulation Pump (BWCP) thrust bearings”
To further define the operation and the result of “other flow interference” see below:
The function of the BWCP is to take water, supplied by the boiler drum, increase its pressure, and pump it down to the lower waterwall headers which distribute the liquid, through a series of orifices, to the furnace waterwalls. The pump and the orifices ensure that all the tubes are adequately supplied, and this process is known as Forced Circulation. It allows the operator to take the boiler to full load much faster than Natural Circulation, which depends on the natural buoyancy of the fluid due to increased temperature to provide boiler circulation.
For proper operation, the pump must receive an uninterrupted flow of water from the boiler drum. If the drum allows steam to be entrained with the liquid (i.e. two-phase flow), the mixture interrupts the normal operation of the impeller when it reaches the impeller. This is commonly called “breaking suction” and it causes very violent axial motion to the whole rotating assembly, which can cause catastrophic thrust failure. There have been instances where the impeller thrusts into the pump casing, causing total failure of all thrust components and bearings as well as severe pump case damage. Also, catastrophic failures normally cause the rotor to touch off on the stator, causing lamination damage to the rotor lams and stator lams, thus requiring a rewind as well as full or partial rotor and stator restacks. Low drum level, as well, can cause axial motion to the rotor.
To conclude, anything that causes the rotor to move up and down or to be sideloaded (cavitation, vibration, entrained steam in the suction, etc.) can, and most likely will, cause thrust and journal bearing wear and eventually motor failure. It is also vital to note, with the removal of the “discharge check valve plugs” the process of filling and purging the unit correctly and ensuring that air is actually out upon starting the motor becomes difficult to determine. It is important to note that air bubbles can be trapped in the winding cavities and if the motor is not filled and purged correctly they can cause premature bearing damage. Additionally, the possibility always exists, that without the heck valve plugs, reverse rotation can occur which can cause reverse thrust loads, causing damage to the reverse thrust seat and plate as well as side loading on the journal bearings.
If you are suffering from Cavitation, Hayward Tyler has designed a retrofit solution. Find out more below or contact us using the form.
